We propose to investigate the molecular structures of the biotin vitamers in order to elucidate the molecular-level events of the biotin carboxylation and transcarboxylation half-reactions. As a working hypothesis we postulate that: (a) the reactivity of biotin in the half-reactions is related to the unusual geometry of the ureido ring; (b) the geometry of the ureido ring is influenced by the overall conformation of the bicyclic cis-3,4-ureylene-thiophane ring system; and (c) the bicyclic ring conformation is a function of the heteroatom in the 1-position and the influence of the valeryl chain. We specifically propose to determine the crystal structures, by X-ray diffraction techniques, of four principal classes of vitamers: (1) heterobiotins (with sulfur replaced by other heteroatoms), (2) bicyclic ring models devoid of side chain, (3) N-carboxybiotins, and (4) sulfur/chain-modified biotins. These studies will complement our already completed determinations of the structures of the natural vitamer, d-(plus)-biotin, and its biological precursor dethiobiotin. We further propose to analyze the geometric and conformational data obtained in these studies in the light of what is already known about the chemistry and biological consequences of the two half-reactions. It is anticipated that these analyses will confirm our working hypothesis that, in essence, the unique reactions of biotin are related to the unique molecular structure of the vitamin. The vehicle for conformational analysis will be the sophisticated software package resident on PROPHET, the NIH-supported computing network for the study of the biomolecules. Because the biotin-dependent enzymes are probably architecturally and mechanistically similar, i.e., they are generally composed of non-identical subunits which catalyze the two half-reactions, and the half-reactions are invariant from enzyme to enzyme, the study of the biotin vitamers takes on an added significance. Our results will be relevant not only to one enzyme systems, but rather to a large family (10 or more) of enzymes. Furthermore, these biotin-dependent enzymes play critical roles at various junctions of the metabolic pathways, controlling fatty acid synthesis, gluconeogenesis, and purine and amino acid synthesis.